Low grade heat, which is typically considered less than 100 degrees, represents a significant waste energy stream in industrial processes, power generation and transport applications. Recovery and re-use of such waste streams is desirable. An example of a technology which has been proposed for this purpose is a Thermoelectric Generator (TEG). Unfortunately, TEGs are relatively expensive. Another largely experimental approach that has been proposed to recover such energy is the use of Shape-Memory Alloys.
A shape-memory alloy (SMA) is an alloy that “remembers” its original, cold-forged shape which once deformed returns to its pre-deformed shape upon heating. This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems.
A heat engine concept is under development which utilises Shape Memory Alloy (SMA) or another Negative Thermal Expansion (NTE) material as the working medium. In such an engine, for example as disclosed in PCT Patent Publication number WO2013/087490 and assigned to the assignee of the present invention, the forceful contraction of such material on exposure to a heat source is captured and converted to usable mechanical work.
Thus far, a useful material for such a working mass has been found to be Nickel Titanium alloy (NiTi). This alloy is a well known Shape-Memory Alloy and has numerous uses across different industries.
For example, NiTi wires form the working element of the engine. Force is generated through the contraction and expansion of these elements within the working core, via a piston and crank mechanism. The most important aspect of this system is the ability to secure the NiTi elements at both ends such that a strong and reliable union is created, enabling high-force, low displacement work to be performed for a maximum number of working cycles.
Nickle Titanium alloy is considerably harder and tougher than the steel material that might be used in the bundle holder. A problem is therefore that the NiTi elements wear or otherwise fatigue the steel elements at the points at which they are in contact. In the case of a friction fit bundle, this would be the boundary at which the outermost NiTi wire elements are located. In the case of “salt shaker” bundle holders, this would be the steel boundary point of contact for each wire hole.
It is therefore an object of the invention to provide a device and method to overcome the above mentioned problem.